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Topic Review (Newest First)

07-12-2010 08:46 PM

cobalt327

There is more to airflow than just quantity.

A high RPM engine will often "like" a bigger carb than the formulas call for, because it delivers a better quality of air/fuel than a small carb will.

If the flow is TOO fast through the venturi, the carb will go pig-rich and the a/f mixture will be bounced off of the plenum floor, etc. This causes all sorts of distribution and a/f separation issues.

07-12-2010 08:29 PM

kleen56

Wow, interesting statement by DoubleVision. I always heard growing up and at street races, that bigger is better including CFM on carbs. Friends use to run a 720 Holley on a 289 Ford and the engine would run like nobodies business. I'm running a 650 DP Holley on a SBF that requires only 480 CFM, The engine runs well under all conditions. I did have to drop the jet sizes a bit but that was it. I get good mileage as long as I don't open the secondaries.

07-12-2010 08:17 PM

DoubleVision

Carbs work on the same principle. Lets say a carb is rated at 700 CFM. We stick it on a high performance 350. We do tests and find the engine only wants 585. The engine runs perfect and has great power, but since the engine only demands 585 should we swap the carb for a smaller one? not really, why is because a carb will only feed a engine what it demands.
Some believe because it`s flow rate is 700 means at wide open throttle it`s flowing 700 like it or not and that`s not the case. If the engine can`t pull it in then it`s not being used. In the real world, Engine demand figures and carb sizes are out to lunch. Even during the horsepower wars of the 50`s and 60`s a 283 with power pack heads that had puny 1.71 valves but yet had a good sized cam, high compression and other goodies came with two four barrels, what was the chance of a 283 using that much CFM? not much.